Endothermic particulate hydroxides have been identified as commercial or potentially commercial fire retardants in polymers, paper and other matrices in the literature. However, these materials have temperature limitations which limit the variety of polymer systems within which they can be used. These temperature limitations include the temperature in which the polymer system is processed and the temperature in which the fire retardant additive begins to decompose and act as a fire retardant.
For example, aluminum hydroxide (alumina trihydrate) is a well-known endothermic fire retardant additive or filler which has been found to be effective in several important polymer systems. It is commonly used at loadings of up to 75 weight percent of the polymer. Although alumina trihydrate is effective, its use is limited to polymers that are processed at temperatures below 220.degree. C., the point where its water of hydration begins to evolve. At processing temperatures above about 220.degree. C., alumina trihydrate decomposes to alumina and water vapor. The water vapor produces unacceptable surface defects and porosity in the finished product, and can damage processing equipment. Water vapor and/or steam which builds up in the processing equipment can cause both mechanical failure and injury to equipment operators.
It is the heat capacity and water of hydration of aluminum hydroxide that makes it effective as a fire retardant additive. To maintain burning in plastic systems, three ingredients need to be supplied: heat, fuel and oxygen.
Alumina trihydrate is effective as a fire retardant because its endothermic decomposition acts as a cooling heat sink to remove heat from the plastic--heat which would ordinarily go toward decomposing the plastic into the low molecular weight gaseous elements needed to sustain combustion. Water liberated from the decomposition of the hydrate also serves to inhibit the access of oxygen to the plastic systems. Thus, the addition of alumina trihydrate transforms an exothermic, fire-propagating polymer system into an endothermic, fire-retarding system.
Some of the common polymers for which alumina trihydrate finds use as a flame retardant include polyurethane, polyethylene, varieties of polypropylene having low processing temperatures and some unsaturated polyesters. Alumina trihydrate is compatible with these polymers because the endothermal decomposition temperature of the alumina (about 220.degree.-300.degree. C.) is above their processing temperatures.
Alumina trihydrate is not presently used as a flame-retardant additive in several other classes of polymers, including for example polyethylene terephthalate (PET), polybutylene-terephthalate (PBT), acrylonitride-butadienestyrene(ABS), nylon, and varieties of polypropylene having high process temperatures. When the trihydrate is added to such polymers, the trihydrate can decompose to alumina and water vapor during processing. The water vapor produces foaming or voids in the polymer matrix. These voids produce surface defects which are unacceptable for most final products.
Many other inorganics possessing "water of hydration" have not been widely used as fire retardants because their water releasing endothermic reactions occur at too low a temperature. One such material is hydrotalcite which is a magnesium aluminum carbonate hydroxide material having the general formula: Mg.sub.x Al.sub.2 (OH).sub.2x+4 (CO.sub.3).yH.sub.2 O where x varies from 3 to 6 and y varies from 2 to 4. Hydrotalcite has the potential of being a good fire retardant because it possesses water of hydration like hydrated alumina. However, it has not found widespread commercial acceptance as a fire retardant because it has several low temperature endotherms. The first of its endotherms can begin at about 50.degree. C.
Attempts have been made in the past to calcine hydrotalcite at a temperature above the processing temperature of the polymer system in which it will be used. The calcined hydrotalcite could then be used as a fire retardant additive which releases its water of hydration at temperatures above the processing temperature of the polymer and below the decomposition temperature of the polymer. These attempts have not been very successful because the calcined material quickly rehydrates by pulling hydroxyl and carbonate groups from ambient air. Thus the calcined hydrotalcite must either be used immediately after calcining or stored in a manner that will prevent it from reverting back to its original composition. Neither of these alternatives has been found to be commercially viable.
There currently exists a need for low-cost additive materials that can be used as fire retardants in polymer systems processed at higher temperature. There also exists a need for low-cost materials that have endotherms which begin at temperatures that are either higher or lower than alumina trihydrate. The endotherms of these materials must be matched to the processing temperatures and/or exothermic characteristics of each polymer system involved.
In addition, a need exists for a method of converting materials that evolve gas at temperatures which are detrimental to the processing of polymer systems, such as hydrotalcite, into materials that do not evolve gas at temperatures which are detrimental to the processing of polymer systems and possess a fire retarding endotherm below the decomposition temperature of the polymer.
The principal object of the present invention is to provide additive material for use as a fire retardant in polymer, paper and other matrices.
Another principal object of the invention is to provide a method of converting materials that evolve gas at temperatures which are detrimental to the processing of polymer systems, such as hydrotalcite, into materials that do not evolve gas at temperatures which are detrimental to the processing of polymer systems and possess a fire retarding endotherm below the decomposition temperature of the polymer.
Another object of the invention is to provide an additive material for use as a fire retardant in polymer systems which possesses enhanced thermal stability.
Another object of the invention is to provide a method of increasing the fire resistance of a known fire retardant by increasing the magnitude of its endotherm.
A further object of the invention is to provide an additive material for use in flame-retardant polymer systems which possesses enhanced thermal stability at higher temperatures.
Still another object of the invention is to provide a method of increasing the usefulness of known fire retardants by changing the temperature at which they begin to act as fire retardants.
Another object of the present invention is to provide a method of creating a new additive material for use as a fire retardant in polymer systems which is designed to meet the specific needs of that polymer system.
Additional objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.